1. Field of the Invention
This invention relates generally to drilling wellbores and more particularly to a drilling system utilizing a downhole pressure intensifier for jet-assisted drilling.
2. Background of the Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached to a drill string. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations.
Modern directional drilling systems generally employ a drill string having a drill bit at the bottom that is rotated by a drill motor (commonly referred to as the "mud motor"). A plurality of downhole devices are placed in close proximity to the drill bit to measure certain downhole operating parameters associated with the drill string and to navigate the drill bit along a desired drill path.
Positive displacement motors are commonly used as mud motors. U.S. Pat. No. 5,135,059, assigned to the assignee hereof and which is incorporated herein by reference, discloses a downhole drill motor that includes a power section having a housing, a stator having a helically-lobed inner elastomeric surface secured within the housing and a rotor having a helically-lobed exterior metallic surface disposed within the stator. Pressurized drilling fluid (commonly known as the "mud" or "drilling mud") is pumped into a progressive cavity formed between the rotor and stator. The force of the pressurized fluid pumped into the cavity causes the rotor to turn in a planetary-type motion. A suitable shaft connected to the rotor via a flexible coupling compensates for eccentric movement of the rotor. The shaft is coupled to a bearing assembly having a drive shaft (commonly referred as the "drive sub") which in turn rotates the drill bit attached thereto. Radial and axial bearings in the bearing assembly provide support to the radial and axial movements of the drill bit. For convenience, the power section and bearing assembly are collectively referred to herein as the "motor assembly." Other examples of the drill motors are disclosed in U.S. Pat. Nos. 4,729,675, 4,982,801 and 5,074,681, the disclosures of which are incorporated herein by reference.
For drilling in rock, the assistance of a jet of high pressure fluid facilitates the drilling operation. Some of the current operations supply the high pressure directly from the surface by either generating the high pressure for the entire fluid flow or operating a smaller amount of high pressure fluid via additional conduits inside the drill pipe. These prior art high pressure systems utilize high pressure pumps or pressure intensifiers at the surface. These systems are relatively expensive and unreliable and thus have not gained commercial acceptance.
The present invention addresses the above-described problems with the prior art methods for jet-assisted drilling and provides novel apparatus and methods for generating high pressure fluid flow downhole.